Hydraulic fluid control apparatus

ABSTRACT

In the field of avionics, it is difficult to justify initiation of new aircraft programmes for replacement of small numbers of niche aircraft. Instead, in many cases, retrofitting existing aircraft is preferred for economic reasons. In this respect, upgrading existing braking systems to accommodate replacement of existing brake units employing sintered friction materials with brake units employing carbon-carbon composite friction materials is desirable. However, when upgrading the braking systems it is necessary to ensure that the integrity of the existing airframe is not compromised by the use of the new brake assemblies employing the carbon-carbon composite friction materials. Consequently, the present invention replaces existing pressure control valves of existing braking systems with an electronic control unit coupled to a pressure control servovalve, thereby ensuring proper application of hydraulic pressure to brake assemblies. An advantage of this invention is that the pressure profile of the hydraulic fluid is controllable and hence the integrity of the airframe is maintained.

The present invention relates to a hydraulic fluid control apparatus ofthe type used, for example, in a braking system for an aircraft.

The high cost of aircraft development means that development programsfor new aircraft need high production numbers and/or selling price tojustify investment levels required. In the aviation industry, there aremany niche applications for small numbers of aircraft, making investmentin a new aircraft difficult to justify. However, aging aircraft thatcurrently fill many of these niche functions are operating withequipment that can be obsolete or inefficient, because of the outdatedtechnology employed. Such equipment can include engines and ancillaryequipment, avionics, landing gear and control systems.

An increasing trend in aviation is the updating of proven airframedesigns with modern equipment to provide an updated, more efficientaircraft that utilises current technology without the cost associatedwith development of a completely new aircraft. Some older airframes thatare now being refurbished and re-equipped for a new life span wereoriginally designed for braking systems employing sintered frictionmaterials. These aircraft are now being equipped with modern brakingsystems. An example of such an aircraft is the Nimrod MRA4 built by BAESystems Limited of Great Britain. However, one drawback of this approachis the accommodation of the performance parameters of new modern systemsby the existing airframe, in particular the torque generated by abraking system.

In this respect, the modern aircraft braking systems are designed aroundthe use of carbon-carbon composite friction materials that have asignificantly higher peak torque during a braking cycle than sinteredmaterials. Consequently, if the brake torque builds too quickly and/orexceeds certain threshold values when the brake is applied, it ispossible to cause damage to the airframe.

Brake control on aircraft employing such airframes refurbished withbraking systems using carbon-carbon composite friction materials hasbeen carried out by the use of complex systems within hydro-mechanicalbrake pressure control valves to control brake torque and avoid damageto the airframe. Typically, a pressure reducing valve, under the controlof a utilities systems management system, feeds hydraulic fluid underpressure to brake metering valves, anti-skid valves and hydraulic fusesdownstream of the pressure reducing valve. The pressure reducing valvebuilds a pressure profile in response to a brake signal received by thepressure reducing valve, the pressure to the brakes being increased overa pre-determined time from a starting pressure to some peak value duringbraking until the pressure is then released after the required brakeapplication has been completed. The functionality of the pressurereducing valve is provided mechanically by, for example, the use of arestrictor to limit the flow of hydraulic fluid and an accumulator, abuild-up of pressure being dependent on the restrictor size andaccumulator volume.

According to a first aspect of the present invention a hydraulic fluidcontrol apparatus for replacing a pressure reducing valve of a brakingsystem, the apparatus comprising: a processing unit for receiving aninput signal and generating an electrical control signal in responsethereto; and means for variably controlling a hydraulic output inresponse to the electrical control signal, the means for variablycontrolling the hydraulic output being coupled to the processing unit.

Preferably, the first control signal corresponds to a demand, when inuse, for braking. More preferably, the processing unit is arranged togenerate the control signal within predetermined parameters irrespectiveof the input signal corresponding to the demand for braking.

Preferably, the control signal is configurable.

The processing unit and/or the means for variably controlling thehydraulic output may be powered by the input signal.

The means for variably controlling the hydraulic output may be aservovalve. The means for variably controlling the hydraulic output maybe arranged to control, when in use, hydraulic pressure and/or hydraulicflow.

According to a second aspect of the present invention, there is provideda braking control system comprising the hydraulic fluid controlapparatus as set forth above in relation to the first aspect of theinvention.

According to a third aspect of the present invention, there is provideda vehicle comprising the hydraulic fluid control apparatus as set forthabove in relation to the first aspect of the invention.

According to a fourth aspect of the present invention, there is providedan aircraft comprising the hydraulic fluid control apparatus as setforth above in relation to the first aspect of the invention.

According to a fifth aspect of the present invention, there is provideda method of upgrading a braking system, the method comprising the stepsof: fluidly coupling between a source of hydraulic fluid under pressureand a braking assembly a means for variably controlling a hydraulicoutput in response to an electrical control signal, the means forvariably controlling the fluid output being provided in place of apressure reducing valve; providing a processing unit for receiving, whenin use, an input signal and generating the electrical control signal inresponse thereto, and coupling the processing unit to the means forvariably controlling the hydraulic output; wherein the processing unitis arranged to control, when in use, supply from the means for variablycontrolling the hydraulic output in response to the control signal.

It is thus possible to provide a hydraulic fluid control apparatushaving reduced complexity of mechanical components, whilst delivering arequired progressive increase in pressure up to a defined maximumpressure. Additionally, by generating a progressive increase in pressureand maximum pressure limit that overrides a rate of brake demand calledby a pilot of an aircraft within limits that are calculated to beacceptable to an airframe, improved safety is achieved. A furtherbenefit of the above apparatus is additional flexibility to be able toadjust a characteristic ramp rate by reselection of electroniccomponents and/or reprogramming of one or more component, and to adjustthe pressure levels generated at the servovalve current limits byadjustment of the apparatus. Furthermore, it is not necessary to provideadditional sources of electrical power in order to drive the apparatus.

At least one embodiment of the invention will now be described, in anon-limiting manner by way of example only and with reference to thefollowing drawings, in which:

FIG. 1 is a schematic diagram of a braking system comprising anapparatus constituting an embodiment of the invention; and

FIGS. 2A to 2D are schematic diagrams of signal and pressure profiles.

Referring to FIG. 1, a braking system 100 comprises a source ofhydraulic pressure 102 coupled to a Shut-Off Valve (SOV) 104, the SOV104 being coupled to a Pressure Control Unit (PCU) 106. The PCU 106comprises a suitably programmed processing unit (not shown) coupled to aUtility Systems Management System (USMS) 110 via an input 108. However,it should be appreciated that the programmed processing unit can bereplaced by other electronic circuitry and/or software. A hydraulicfluid output port 112 is coupled to a first Brake Metering Valve (BMV)114 and a second BMV 116 by a brake systems hydraulic line 113, thefirst and second BMVs 114, 116 being coupled to a first set of Anti-SkidValves (ASVs) 118 and a second set of ASVs 120, respectively. Each ASV120 is coupled to a corresponding brake assembly 122 via a respectivehydraulic fuse 124.

The PCU 106 comprises an Electronic Control Unit (ECU) 126 having aninput constituting the input 108 of the PCU 106, and an output coupledto an input of a Pressure Control Servovalve (PCS) 128. The PCS 128 hasan input port 130, constituting a hydraulic fluid input port of the PCU106, and an output port constituting the hydraulic fluid output port112.

In this example, the ECU 126 and the PCS 128 are formed as a singleunit, namely the PCU 106. However, it should be appreciated that the ECU126 and PCS 128 can be provided as separate units.

In operation (FIG. 2), a braking demand originates, for example, from apilot. The braking demand is translated into an electrical brakingdemand signal. The braking demand signal is received by the USMS 110 inaddition to other signals representative of factors pertinent tobraking, for example: aircraft weight and/or speed. In this respect, theUSMS 110 executes a number of algorithms in order to generate a braketrigger signal 200 (FIG. 2A) that is received by the ECU 126 via theinput 108. In this example, the PCU 106 derives electrical power fromthe brake trigger signal 200.

In response to the brake trigger signal 200, the ECU 126 processes thebrake trigger signal 200 using a suitable control algorithm stored inthe ECU 126 in order to generate a brake pressure demand signal 202(FIG. 2B) that is received by the PCS 128. Upon receiving the brakepressure demand signal 202, the PCS 128 actuates in accordance with thebrake pressure demand signal 202 to apply pressure to the brake systemhydraulic line 113 via the output port 112; the pressure profile appliedto the braking system hydraulic line 113 follows a predeterminedpressure vs. electrical input signal profile 204 (FIG. 2C) of the PCS128 to yield a brake pressure 206 (FIG. 2D).

Consequently, the brake assemblies 202 effect braking to slow theaircraft within acceptable mechanical parameters of the airframe of theaircraft, thereby avoiding compromising integrity of the airframe.

If desired, the profile of the brake pressure demand signal 202, andhence the profile of the brake pressure 206, can be easily modified byre-programming the ECU 126 and/or modifying at least one component ofthe ECU 126.

In this example, the ECU 126 is programmed so that, irrespective of thebrake demand made by the pilot, the translation of the brake demand bythe pilot into the brake pressure 206 is overridden, when necessary, bythe ECU 126 in order to avoid the profile of the brake pressure 206deviating outside, or crossing one or more threshold corresponding to,predetermined pressure profile limits, thereby maintaining airframeintegrity if the pilot issues an unacceptable braking demand.

1-11. (canceled)
 12. An aircraft braking system hydraulic fluid controlapparatus for replacing a pressure reducing valve of an aircraft brakingsystem, the apparatus comprising: electrical control circuitry forreceiving an input signal representative of a brake demand andgenerating an electrical control signal in response thereto; and meansfor variably controlling a hydraulic output in response to theelectrical control signal, the means for variably controlling ahydraulic output being coupled to the electrical control circuitry forreceiving said electrical control signal, wherein said electricalcontrol circuitry is operable to override said input signal and generatean electrical control signal to cause said means for variablycontrolling a hydraulic output to provide a predetermined hydraulicoutput profile according to airframe properties of an aircraft to which,in use, the apparatus is fitted when a hydraulic output corresponding toa said brake demand would exceed a threshold value associated with saidairframe properties.
 13. An apparatus as claimed in claim 12, whereinsaid electrical control circuitry comprises an electronic control unit.14. An apparatus as claimed in claim 13, wherein said electronic controlunit is a programmable processing unit, whereby the electrical controlsignals generated are configurable by reprogramming the processing unitsuch that the hydraulic output from the means for variably controlling ahydraulic output can be adjusted by reprogramming the processing unit.15. An apparatus as claimed in claim 12, wherein the electrical controlcircuitry is an open loop control that generates said electrical controlsignal without feedback concerning the hydraulic output.
 16. Anapparatus as claimed in claim 12, wherein at least one of the controlcircuitry and the means for variably controlling a hydraulic output ispowered by the input signal.
 17. An apparatus as claimed in claim 12,wherein the means for variably controlling a hydraulic output is aservovalve.
 18. An apparatus as claimed in claim 12, wherein the meansfor variably controlling a hydraulic output is arranged to control, whenin use, at least one of hydraulic pressure and hydraulic flow.
 19. Anaircraft braking system comprising an aircraft braking system hydraulicfluid control apparatus as claimed in claim
 12. 20. An aircraftcomprising an aircraft braking system as claimed in claim
 19. 21. Amethod of upgrading an aircraft braking system, the method comprising:coupling a means for variably controlling a hydraulic output in responseto an electrical control signal between a source of hydraulic fluidunder pressure and a braking assembly of an aircraft braking system, themeans for variably controlling a hydraulic output being provided inplace of a pressure reducing valve; providing electrical controlcircuitry for receiving, when in use, an input signal representative ofa brake demand and generating the electrical control signal in responsethereto; and coupling the electrical control circuitry to the means forvariably controlling a hydraulic output; wherein the means for variablycontrolling a hydraulic output is arranged to control, when in use,supply from the source of hydraulic fluid in response to electricalcontrol signals from said electrical control circuitry.
 22. A method asclaimed in claim 21, wherein said electrical control circuitry comprisesa programmable processing unit and further comprising programming saidprogrammable processing unit to generate electrical control signals thatwill cause the means for variably controlling a hydraulic output tocontrol supply from the source of hydraulic fluid to provide pressureprofiles that are within a predetermined pressure profile limitacceptable to an airframe of an aircraft to which the braking system isfitted even when the brake demand is for a pressure profile exceedingsaid predetermined pressure limit.
 23. A method as claimed in claim 22,wherein said programming comprises providing a pressure profile having acharacteristic ramp rate acceptable to the airframe.
 24. A method asclaimed in claim 22, wherein said programming comprises providing apressure profile having a maximum pressure acceptable to the airframe.25. An aircraft braking system comprising a braking assembly actuable byhydraulic fluid supplied by a hydraulic pressure source and a controlapparatus disposed upstream of the braking assembly for receivinghydraulic fluid from the hydraulic pressure source and providing acontrolled hydraulic output to the braking assembly, said controlapparatus comprising: a valve responsive to electrical control signals;and an electrical control operatively connected with said valve forproviding said electrical control signals to the valve in response toreceived braking demand signals, said electrical control being arrangedto issue electrical control signals that cause said valve to operate toprovide a said controlled hydraulic output that does not exceed apredetermined pressure profile limit irrespective of braking demandsignals received by said electrical control.
 26. An aircraft brakingsystem as claimed in claim 25, wherein said valve is a servovalve. 27.An aircraft braking system as claimed in claim 25, wherein saidelectrical control comprises an electronic control unit.
 28. An aircraftbraking system as claimed in claim 27, wherein said electronic controlunit comprises a programmable processor and said predetermined pressureprofile limit is determined by airframe parameters of an aircraft, saidprocessor being programmable to permit programming of respectivepredetermined pressure profile limits determined by airframe parametersof different aircraft.
 29. An aircraft braking system as claimed inclaim 25, wherein said electrical control is an open loop control thatprovides said electrical control signals without feedback.
 30. Anaircraft system as claimed in claim 25, wherein at least one of saidelectrical control and said valve is powered by said braking demandsignals.
 31. An aircraft braking system as claimed in claim 25, whereinsaid braking assembly comprises carbon-carbon composite frictionsurfaces.
 32. An aircraft braking system as claimed in claim 25, whereinsaid electrical control comprises software.
 33. An aircraft fitted withan aircraft braking system as claimed in claim
 25. 34. A method ofupgrading an aircraft braking system, the method comprising: replacing afirst braking assembly with a second braking assembly capable ofdelivering a higher brake torque than the first braking assembly, saidsecond braking system being actuable by hydraulic fluid received from ahydraulic pressure source; and providing a control apparatus forcontrolling the supply of hydraulic fluid from said hydraulic pressuresource to the second braking assembly, said control apparatus comprisinga valve responsive to electrical control signals and an electricalcontrol for receiving brake demand input signals and providing saidelectrical control signals in response to said brake demand signals,said electrical control being arranged to issue electrical controlsignals that cause said valve means to operate to provide a saidcontrolled hydraulic output that does not exceed a predeterminedpressure profile limit irrespective of the brake demand signals receivedby said electrical control.
 35. A method as claimed in claim 34, whereinsaid second braking assembly comprises carbon-carbon composite frictionmaterials.
 36. A method as claimed in claim 34, wherein said electricalcontrol comprises a programmable processor and further comprisingprogramming said processor with a predetermined pressure profile limitdetermined by airframe parameters of an aircraft with which the secondbraking system is to be used.
 37. A method as claimed in claim 34,wherein said electrical control comprises an electronic control unit andfurther comprising selecting at least one component of the electroniccontrol unit to provide said predetermined pressure profile limit.
 38. Amethod as claimed in claim 34 wherein said predetermined pressureprofile limit comprises a maximum pressure value.
 39. A method asclaimed in claims 34, wherein said predetermined pressure profilecomprises a maximum ramp rate.
 40. A computer program product comprisingat least one software element which, when executed in an executionenvironment is operable to implement said generation of electricalcontrol signals in the method claimed in claim 34.